We review online activism and its relations with offline collective action. Social media facilitate online activism, particularly by documenting and collating individual experiences, community building, norm formation, and development of shared realities. In theory, online activism could hinder offline protests, but empirical evidence for slacktivism is mixed. In some contexts, online and offline action could be unrelated because people act differently online versus offline, or because people restrict their actions to one domain. However, most empirical evidence suggests that online and offline activism are positively related and intertwined (no digital dualism), because social media posts can mobilise others for offline protest. Notwithstanding this positive relationship, the internet also enhances the visibility of activism and therefore facilitates repression in repressive contexts.